In the $20 billion mobile display market, the dominant display technology is liquid crystal (LCD) technology. This dominance may be measured by the fact that in 2006, of the 1 billion plus displays that were shipped for mobile devices, such as cellular phones, over 95% used LCD technology.
Despite their present dominance, LCD displays have the drawback of requiring back-lighting that can be very power consuming. This is of particular concern in battery operated, mobile devices such as cellular phones. In a typical cellular phone, back-lighting consumes about 80% of the battery power required to operate the phone.
Because of this drawback, there is considerable interest in alternative display technologies, particularly reflective display technologies that make use of ambient light rather than requiring backlighting. One alternative, reflective display technology that has received considerable attention is electrowetting display technology.
Electrowetting display technology uses the well-known electrowetting effect, in which the wetting properties of a surface can be altered by applying a voltage, to manipulate a colored oil from covering an entire picture element to beading up to cover only a small fraction of the picture element. When the oil covers the entire picture element, all the light is absorbed by the dye coloring the oil and the picture element is in an “Off” state. When the oil covers only a portion of the picture element, light can be transmitted—or reflected—by the picture element and it is in an “On” state. Reflective electrowetting displays may be made in which no light is reflected in the “Off” state and about 65% of the incident light is reflected in the “On” state. This compares to paper in which white paper reflects about 70% of the incident light, while black ink reflects almost none.
Electrowetting display technology has been described in detail in, for instance, US Patent publications no. 20050270672 by Feenstra et al. (Dec. 8, 2005), no. 20050213014 by Feenstra et al. (Sep. 29, 2005), no. 20050104804 by Feenstra et al. (May 19, 2005) no. 20060132404 by Hayes et al. (Jun. 22, 2006), no. 20050123243 Steckl et al. (Jun. 9, 2005) no. 20070031097 by Heikenfeld et al. (Feb. 8, 2007) the contents of all of which are hereby incorporated by reference.
FIGS. 1A and 1B show various components of an exemplary embodiment of prior art electrowetting display technology. A prior art, electrowetting picture element 10 typically has two immiscible fluids contained between an upper, hydrophilic surface 16 and a lower, hydrophobic surface 18. The immiscible fluids are typically a non-polar liquid 12, such as oil, and a polar liquid 14, such as water. The non-polar liquid 12 typically has added dyes or pigments to facilitate absorbing some, or all, of the incident light.
In an “Off” state, shown in FIG. 1A, the non-polar liquid 12 completely covers the hydrophobic surface 18 of the electrowetting picture element 10. In this “Off” state, incident light 22 is absorbed by the non-polar liquid 12 and any added dyes or pigments it may contain.
An “On” state of the prior art, electrowetting picture element 10 is achieved by applying a suitable voltage 24 between a transparent electrode 20 and the polar liquid 14. The transparent electrode 20 is electrically isolated from the polar liquid 14 by the hydrophobic surface 18 that is also a dielectric. The polar liquid 14 may have additives such as, but not limited to, acids, alkalis or salts or a combination thereof, to make the polar liquid 14 more conductive to electricity. The application of a suitable voltage 24 to such an arrangement results in the well-know electrowetting phenomenon in which the surface 18 becomes less hydrophobic. As surface 18 becomes less hydrophobic, the non-polar liquid 12 beads up, allowing the polar liquid 14 to come into contact with surface 18. As a result, some of the incident light 22 is no longer blocked by the non-polar liquid 12 and now emerges from the prior art, electrowetting picture element 10 as emergent light 26.
A significant draw back of the prior art, electrowetting picture element elements 10 are that they require a small, but continuous, voltage to be applied to maintain an “On” state. This means that when they are displaying an image, they are consuming power.
For many applications, including e-books, e-signage and mobile device displays, it is desirable to have both an “On” state as well as an “Off” state that require no power. Such a display element is typically termed a “bistable” display element.